Combine chile powder, salt, pepper, cumin, oregano, & garlic powder in a small bowl.
Rub spice mixture all over the chuck roast. Place onion in the bottom of a crock pot, place chuck roast on top of onion and pour beef broth in. Cook on high for 6 to 7 hours. Shred with two forks and set aside.
Place poblanos and garlic cloves on a cookie sheet and roast under broiler until garlic is browned (you will have to remove these before the peppers are done) and peppers are blistered.
Let peppers cool in a pot with a lid on it and peel skin off. Cut a slit on one side, scoop seeds out and run under cool water to remove any extra seeds not removed.
Combine the can of tomato sauce, garlic, onion and 1/4 cup beef juice(both scooped out of crock pot) in a blender until smooth.
Stuff chiles with 1/4 cup shredded beef, jack cheese, top with tomato sauce, more cheese and bake in the oven at 425*F degrees for 15 to 20 minutes.